The invention relates to electron multipliers (EM), including continuous surface and discrete dynode multipliers and magnetic electron multipliers. In particular, the invention relates to channel electron multipliers (CEM) and CEM assemblies such as microchannel plates (MCP) which have reduced ion feedback.
Channel electron multipliers are tubular structures and are commonly fabricated from a special formulation of glass, which is heavily lead-doped. When properly processed, the glass exhibits useful secondary emissive and resistive characteristics.
Known CEMs exhibit end-to-end resistances in the range of 10.sup.7 to 10.sup.9 ohms. Electrical contacts, usually Nichrome, are deposited on both ends of the channel. This allows good electrical contact between an external voltage source and the CEM. The external voltage source serves a dual purpose. First, the channel wall replenishes its charge from the voltage source. Second, the applied voltage accelerates the low energy secondary electrons in the channel to a level where, upon collision with the surface, they create more secondary electrons. Electron multiplication or gain in excess of 10.sup.8 is possible with CEMs having an inside diameter of about 1 millimeter or less.
A straight channel electron multiplier 20 of the prior art is shown in FIG. 1. The CEM is a glass tube 21 whose interior surface acquires suitable resistive and secondary emissive properties through treatment of that surface which is sometimes referred to as a secondary emissive layer or interior surface. The ends of the multiplier 20 are coated with an electrode material 24 to which a high voltage potential 26 of a few thousand volts is applied. This operation should be performed in a vacuum of about 10.sup.-6 torr or better. Higher pressure operation increases the ion density in the channels which leads to specious electron pulses. High voltage should not be applied at pressures greater than 10.sup.-4 torr as electrical breakdown of the gas may occur. This usually results in a destroyed multiplier.
An incident particle 28, for example, an electron from an electron source 30 or a photon of sufficient energy is detected when it strikes the secondary emissive layer or interior surface of the CEM 20 and causes the emission of at least one secondary electron 34. The secondary electron 34 is accelerated by the electrostatic field created by the high voltage 26 within the channel 20 until it again hits the interior surface of the channel 20 as shown by the arrows. Assuming it has accumulated enough energy from the field, more secondaries 34 will be released. This process occurs ten (10) to twenty (20) times in a channel electron multiplier, depending upon its design and use, thereby resulting in a significant signal gain or cascade of output electrons 38.
It is of interest to note that the gain of the CEM 20 is not a function of channel length or diameter independently, but rather a function of the length-to-diameter ratio. It is this fact that allows considerable reduction in both length and diameter and hence the fabrication of very small arrays of CEMs called microchannel plates (MCP) which have channel dimensions approximately 100 times smaller than a typical CEM. Unless otherwise noted herein, the characteristics of CEMs and MCPs are similar except that the MCP has multiple channels. Thus, the term channel electron multiplier or its abbreviation CEM is intended to include a microchannel plate.
A microchannel plate 40 illustrated in FIG. 2 begins as a glass tube filled with a solid, acid-etchable core which is drawn using fiber-optic techniques to form single fibers called mono-fibers. A number of these mono-fibers are then stacked in a hexagonal array called a multi. The entire assembly is drawn again to form multi-fibers. The multi-fibers are then stacked to form a boule or billet which is fused together at high temperature.
The fused billet is sliced on a wafer saw to the required bias angle, it is edged to size, and then ground and polished to an optical finish. The individual slice 42 is chemically processed to remove the solid core material, leaving a honeycomb structure of millions of tiny holes 44 which extend at an angle 48 between the faces 49 of the MCP. Each hole or channel 44 is capable of functioning as a single channel electron multiplier which is relatively independent of the surrounding channels.
Through subsequent processing, the interior surface 43 of each channel 44 in this specially formulated glass wafer 42 is given conductive and secondary emissive properties. Finally, a thin metal electrode 50 (usually Inconel or Nichrome) is vacuum deposited on the faces 49 of the wafer 42 to electrically connect all the channels 44 in parallel. High voltage 52 may then be applied across the MCP 40. The fragmented cross-sectional diagram in FIG. 2 illustrates the major mechanical components of all known microchannel plates.
MCPs may be fabricated in a wide variety of formats. The arrays may range in size from 6 millimeters to 150 millimeters or larger and they may be circular, rectangular or virtually any other shape as required by the application or instrument geometry.
For normal operation, a bias voltage 52 of up to about 1000 volts is applied across the microchannel plate 40, with the output at its most positive potential. The bias current flowing through the plate resistance is what supplies the electrons necessary to continue the secondary emission process. This process is similar to that which occurs in the single channel electron multiplier 20 (FIG. 1).
Straight CEMs and MCPs are unstable at gains in excess of 10.sup.4 in the sense that output pulses appear which are not directly caused by input photons or particle incidence. The primary reason for this instability is the phenomenon known as ion feedback which is schematically illustrated in FIG. 1. The number of electrons which move through the CEM 20 increases exponentially towards the output end 54. The same is true for an MCP. In this region, therefore, there is a high probability of ionizing some of the residual gas molecules within the channel 20, which ions 56 are illustrated schematically as an encircled plus sign.
Ion feedback is the process by which many of the residual gas molecules within the channel 20 become ionized by the intense electron flux which exists near the output end 54 of the channel 20. The ions 56 being positively charged are attracted or accelerated towards the input end 58 of the channel 20 due to the potential 26 applied to the device. The motion of the ions 56 is illustrated by dotted arrows. If these ions 56 acquire sufficient energy, secondary electrons 34' will result upon collision with the secondary emissive layer or interior surface of the channel. The ion induced secondary emissions 34' in turn cascade and multiply, leading to spurious output pulses which degrade the performance of the device.
In extreme cases a condition known as regenerative ion feedback or ion runaway can occur in which ion induced secondary electrons 34' multiply and continue to produce ions spontaneously without a primary input 28. In this condition, the device will continue to produce output events long after all input events 28 have stopped.
Ions 56' (and neutral molecules) which escape the channel may impinge on and adversely affect the electron source 30. For example, in a light intensification device the electron source 30 is a photocathode and the phenomenon is generally referred to as ion poisoning.
MCPs and CEMs can operate in two modes. In the first mode, known as the analog mode the electron multiplier is operated as a current amplifier. In this type of operation, the output current increases proportionally to the input current by the product of the gain factor. The output pulse height distribution is characterized by a negative exponential function.
FIG. 3 illustrates the principle by means of a plot which represents the number of pulses or pulse height distribution about an average gain G verses the gain of an analog CEM. A similar characteristic curve results with an MCP. The curve in FIG. 3 is known and is referred to in the art as a negative exponential.
The second mode of operation is known as the pulse counting mode. In this mode of operation the multiplier is operated at a sufficiently high input event level to drive the device into space charge saturation in which sufficient electron densities within the channel create inter-electron repulsive forces which limit the electron gain. The space charge saturation effect gives rise to an output pulse height distribution which is tightly fitted about a modal gain point. This pulse height distribution is approximated by Poisson statistics and is considered Gaussian.
FIG. 4 is a plot of the number of integrated output pulses verses gain in a CEM operating in the pulse counting mode. The plot shows that a pulse counting CEM, which operates at a higher gain, has an output pulse height that has a characteristic amplitude. FIG. 4 is known and is referred to as a Gaussian distribution. In contrast, the analog CEM has an output characteristic which varies widely.
There is an optimum voltage at which to operate a pulse counting CEM. FIG. 5 shows a typical plot of output count rate observed on a counter as a function of CEM applied voltage when the input signal is constant. The output count rate is observed to plateau as the CEM enters saturation (point A, approximately 10.sup.8 gain). For pulse counting it is desirable to operate the CEM about 50 to 100 volts above this point, i.e. at point B. Operation at voltages above this value does not increase the gain very much, but according to the prior art it can have detrimental effect on the device. First, the life of the CEM can be unnecessarily decreased. Second, when operating at voltages far in excess of those necessary for saturation, ion feedback may occur very early in the channel, resulting in noise pulse and possibly regenerative ion feedback. This phenomenon has traditionally been considered to have an extremely detrimental effect on the life and overall performance of CEMs and MCPs. Thus, the prior art has traditionally avoided those conditions which might result in an ion feedback and has particularly avoided the operation of MCPs and CEMs under conditions of regenerative ion feedback.
There are basically two methods for reducing ion feedback: firstly, ion blocking or trapping; secondly, prevention of ion formation. In the first method the probability of ions gaining enough energy or momentum to cause spurious noise is reduced by physical or electrical alteration of the channel. In general, ion trapping or blocking does not remove the source of ion feedback, namely the ions themselves. Ion elimination by the prevention of ion formation is clearly to be preferred.
One known method which greatly reduces ion feedback instability in CEMs and MCPs by ion trapping is a technique in which the channel or channels are curved. Curvature limits distance that an ion can travel towards the input end of the multiplier. Since the highest probability of generating ions exists near the output end of the channel and the distance toward the input that these ions can travel is limited, the gain of pulses due to these ions is very low in comparison to the overall gain of the device. Also, the lesser impact energy of these ions reduces the probability of secondary emission. Elimination of ion feedback allows electron multipliers of appropriate design to operate at gains in excess of 10.sup.8. Even though curved MCPs provide high gain without feedback, curved channel MCPs are difficult to manufacture and are expensive.
Some channel structures are modifications of the curved channel arrangement wherein the channel is helical. Such structures are difficult to produce with uniform characteristics and at reasonable cost.
Some channel structures distort the electric field causing the ions to be driven into the side wall of the channel before achieving sufficient momentum to initiate secondary emission. Such devices include ribbed channels, channels with a glass dike, or MCPs having bulk conductivity. These devices are likewise difficult and expensive to make and hard to control.
Another known method for trapping the ions employs two or more back to back MCPs in so-called Chevron.TM. or Z-stack arrangements. The plates are stacked in such a way that the bias angles of the channels in each adjacent MCP are at an angle to each other so that the ions produced in the output plate are prevented from being fed back to the input plate.
Another method of trapping the ions employs an ion barrier which is an ultra-thin membrane of silicon oxide SiO.sub.2 or aluminum oxide Al.sub.2 O.sub.3 formed on the input side of the plate which is opaque to ions, but is transparent to electrons of sufficient energy. Ion barriers effectively stop ion feedback to the photocathode. However, they do not address the problems of after pulses caused by ion feedback generated within the channel. Ion barriers may also adversely effect the signal to noise ratio of the plate because of the necessity to deliver higher energy incident or primary electrons to the plate which are capable of penetrating the film. The use of an ion barrier also necessitates operating the plate at a higher voltage to thereby provide higher energy primary electrons which higher voltage is not desirable. Collection efficiency is also reduced because most electrons scattered by the film between the channels have insufficient energy to thereafter penetrate the film and interchannel material to result in secondary emissions.
Ion formation is known to be diminished when the EM is operated under various high vacuum and high temperature conditions sometimes called a "bake" or "bake out" followed by electron bombardment degassing sometimes called "scrub": for example, less than 10.sup.-5 torr at 380.degree. C., followed by electron scrubbing at an extracted charge rate of 6.6.times.10.sup.-4 Q/cm.sup.2 per hour for about 24-48 hours at room temperature. The process, employing a high vacuum and high temperature bake followed by room temperature electron bombardment degassing may occur over an extended period of time, for example, from a few hours to months. The so-called "bake and scrub" process in its various forms is time consuming and expensive to implement. In addition, a greater reduction in ion formation is desired.